if you have a requirement for XP-64 feel free to let your account manager know. We have no plans to make it at the moment as the world-wide user base for us is tiny. If you have a lot of machines we might reconsider, and of course if you wanted to pay for a special version, again that can be put in the pot for consideration.
XP64 was pretty niche when it came out, it never really went anywhere once Vista was launched. The non-professional version only works on Itanium processors, and people stopped shipping desktops with those in back in 2005.
Of course you may have a million machines running it so please let us know what kind of volume you need protection for.